The present invention relates to a plasma jet ignition plug, and more particularly to a plasma jet ignition plug for a plasma jet ignition system for an automotive internal combustion engine.
In order to extend the lean misfire limit of the conventional spark ignition internal combustion engines, there is a continuing interest in new ignition sources and their effects on engine performance and emissions. Various kinds of new ignition systems have been proposed.
A plasma jet ignition plug 10, as shown in FIG. 1, has been proposed which comprises a first or rod-shaped electrode 1, a second electrode 2, and an insulating body 3 of a ceramic, for example, which together with the first and second electrodes defines a substantially enclosed plasma cavity 4. The second electrode closes one end of the plasma cavity and is formed with an orifice 5 therethrough. The first rod-shaped electrode 1 extends part-way towards the second electrode 2 whereby to define a plasma cavity gap between said first and second electrodes.
Different from the usual spark plug that directly heats the air fuel mixture up to its ignition temperature, the plasma jet ignition plug 10 generates a spark across the plasma cavity gap, thus generating a high temperature, high energy plasma gas within the plasma cavity 4. This gaseous plasma confined in the plasma cavity 4 is partially ejected though the orifice 5 owing to an increase in pressure of the confined plasma within the plasma cavity 4. The ejected gaseous flow of plasma, with high temperature and high energy, forms many small spot-like flames within the combustion chamber of the engine, thus assuring safe ignition of the air fuel mixture.
Connected to the first electrode 1 of the plasma jet ignition plug 10 is a power source 20 which includes a spark energy storage system 20a and a plasma jet energy storage system 20b.
The spark energy storage system 20a is substantially similar in construction to a conventional ignition system and provides the basic spark timing and high voltage trigger signal to the plasma jet ignition plug 10. It includes a cam 21 which is rotatable in timed relationship with the engine rotation, a breaker 22a with a breaker arm 22, an ignition coil 23 having a primary winding 23a connected to the breaker 22a and a secondary winding 23b, and a battery 24.
The plasma jet energy storage system 20b includes a coil 25, a storage capacitor 26 connected in series with the coil 25, a charging resistor 27, and a power source 28. A steering diode 29 is arranged to prevent the spark energy from flowing into the storage capacitor 26.
In operation, the contact 22a is moved to an open position by the contact arm 22 activated by the cam 21 which rotates in timed relationship with the engine rotation, a primary current passing through the primary winding 23a is interrupted, thus inducing a high voltage on the secondary winding 23b, causing the discharge of a spark to take place within the plasma cavity 4 between the first electrode 1 and the second electrode 2. This spark discharge causes a breakdown of insulation of the plasma cavity 4 so that a discharge of spark within the discharge cavity 4 becomes possible even with a relatively low voltage. Thus the discharge of spark will continue within the discharge cavity owing to the supply of direct current from the plasma energy storage system 20b. That is, all of the energy stored on the capacitor 26, having been charged by the power source 28 via the resistor 27, is discharged via the steering diode 29 and coil 25, thus sustaining the discharge of spark within the plasma cavity 4.
The discharge of a spark by the plasma jet ignition plug generates an electromagnetic wave noise that disturbs radio broadcasting service, television broadcasting service and other kinds of radio communication systems. The noise also causes operational errors in electronic control circuits, for example electronic controlled fuel injection systems and electronic controlled skid control systems. As a result, traffic safety will be threatened.
In order to suppress the noise, there has been proposed to use a resistance plasma jet ignition plug which has an electrode containing a resistance of the order ranging from several K.OMEGA. to 10 K.OMEGA.. This proposal is defective in that an increase in resistivity causes a reduction in plasma jet energy to an unacceptable low level, deteriorating the ignition capability.